Get a pair of ten-sided dice  and decide what dice will mark the tens and what will mark the units. Roll and look in the following table to see what you will be in your next life. Remember to flip a coin to determine sex for the things that have that.
Beware of spoilers
21-65: bacteria (roll on separate table to determine species). Life's very short, but if you like company and are so narcissistic that prefer to clone yourself you'll not be alone and, hey, you can mess with superior lifeforms. Just remember that if you kill them, you and your progeny will die, thus coming here to roll.
66-80: plant (roll on separate table to determine species). Life's very boring (you can't move, can't see, can't hear, etc.) and may be really long. Take it easy.
81-84: insect (roll on separate table to determine species). If you're a beetle, you're in luck: God has an inordinate fondness for you. Life's also short and likely ending in a horrible way (crushed under a boot, killed by a man-made venom, eaten by another animal…). Your mission: to have the largest possible number of descendants.
85-87: worm (roll on separate table to determine species). You'll spend your life — besides copulating — moving under the soil and eating dirt. No, you will not fight other worms with human-designed weapons.
88-89: spider (roll on separate table to determine species). Like an insect, except your diet will mainly consist on liquified insect tissue, plus liquified male spider tissue if you're a female.
91: bird (roll on separate table to determine species). You will be traveling from one place to other once you learn to fly. If you're born as a social bird like a budgerigar, cockatiel, or another similar bird, that is, you won't be able to fly all the time, but it's not a bad trade-off for almost no predators, shelter (which also includes your cage despite what other humans think of it) free food and water, and, with some luck, a mate. And if you're a less social bird like a canary, not being able to fly everywhere isn't a bad trade-off for a predictable and stable environment.
94: cow. If you're in India you'll be respected. Otherwise, as a male pray — or whatever — you're not born on Spain or another country with a bullfighting tradition (your death will be excruciatingly painful, but at least you may take with you that toreador). If you're female, you'll give milk or will become hamburgers.
orc human (duh). Face it, at least it's temporary. Remember to tell others that in your past life(s) you were a Pharaoh, Archduke Franz Ferdinand, or another similarly important person (salesmen, farmer, and other people are not so lucky to reincarnate).
99: goat. Congratulations!.
100: Ur-Quan Kohr-Ah. You're in command of a Marauder-class battleship and it's time to cleanse all those other pesky alien races that are a menace for your existence. Remember to tell them before dying that they may be born again as an Ur-Quan.
(Feel free to add more)
- Or a dice with a hundred sides (yes, those monstrosities exist)